Method, device and computer program product for monitoring the physiological state of a person

ABSTRACT

The invention relates to a method, device, and computer program product for monitoring the physiological state of a person. In the method, the heartbeat of the person is detected in order to obtain a pulse signal, and at least one parameter depicting the respiration of the person is determined in the time domain with the aid of time stamps made of the basis of the pulse signal. With the aid of the method, it is possible to calculate an estimate of the person&#39;s energy consumption during exercise, without complicated calculations or preliminary data based on measurements.

The present invention relates to the measurement and evaluation ofphysiological functions. More specifically, the invention relates to amethod for estimating energy consumption and a device and computerprogram product for implementing the method.

U.S. Pat. No. 6,537,227 discloses one method for estimating energyconsumption. In the method, the heart rate of the person and pre-enteredreference parameters depicting the performance of the person areutilized, on the basis of which the energy consumption duringperformance is estimated. The calculation requires information on theperson's maximum oxygen consumption (VO2max). Estimating this accuratelyprior to performance is not simple, and an erroneous estimation mayproduce a large error in the calculation.

WO publication 2003/099,114 discloses another kind of calculationmethod, in which the respiratory frequency is calculated with the aid ofchanges in the frequency and a modulation function generated through aneural network. Such a calculation requires a large calculation andmemory capacity, which increases the power consumption, size, and priceof the device performing it. Further, in the energy consumptioncalculation described in the publication, which is based on respiratoryfrequency through a change in the frequency, information or an estimateis required on the respiratory volume value of the person, or on acorresponding personal physiological variable, which, in order to createan accurate energy consumption value, should be separately measured, forexample, in an exercise test, for entering to the performing device.

WO publication 2004/073,494 discloses a method for measuring energyconsumption by exploiting the data of an acceleration sensor and aheart-rate meter. US publication 2005/054,938 discloses a method, inwhich further the data provided by an altimeter is utilized.

It is an aim of the invention to create an entirely new type of methodfor monitoring the physiological state, particularly the energyconsumption of a person during exercise. In particular, the method isintended to create a method that can be implemented using a smallercomputing capacity than in the known solutions.

In addition, it is an aim of the invention to create a new portabledevice for monitoring the state of a person during exercise, as well asa new computer program product.

The invention is based on the observation that respiratory frequencyand/or other parameters relating to respiration can be derived directlyfrom the pulse signal using the periodicity of the temporal variation ofthe pulse intervals (pulse interval noise) in the time domain.

In the method according to the invention, the heart rate of the personis measured in order to obtain a pulse signal comprising temporallysuccessive pulse periods, or such measurement data is received from asuitable sensor, and, on the basis of the periodicity of the pulseinterval noise, at least one parameter depicting the respiration of theperson is determined directly in the time domain.

The device according to the invention for monitoring the physiologicalstate of a person comprises means for measuring the heart rate, in orderto detect temporally successive pulse periods, or means for receivingsuch a pulse signal. In addition, the device comprises means fordefining at least one parameter depicting the person's respiration inthe time domain, On the basis of the pulse interval noise of the pulsesignal.

The computer program product according to the invention for defining thephysiological state of a person is arranged to receive measurement datadepicting the heart rate of the person and to determine, on the basis ofthe periodicity of the temporal variation of the heat-rate datacontained in the measurement data, at least one parameter depicting therespiration of the person, on the basis of pulse detections in the timedomain, with the aid of time stamps made.

Considerable advantages are obtained with the aid of the invention.Determination performed in the time domain, compared to analysisperformed through a change in frequency, has the advantage of a reducedneed for calculation. Thus the calculation is rapid and can be performedusing small processor and program memory capacities, so that powerconsumption is also reduced and the device can be made cheaper. Lowpower consumption in turn means a longer operating time For the deviceand/or the possibility to use smaller batteries. Thus it is highlysuitable for portable devices, such as wristop computers. The knownmethods based on frequency analysis typically use Fouriertransformation, which makes the calculation complicated and requiringcalculating power.

By using the invention it is also possible to achieve a sufficientlyaccurate estimate of energy consumption during exercise, without knowingthe person's maximum oxygen or energy consumption. Thus the user neednot perform an exercise test or similar test providing information onthe user's metabolism, before monitoring of their personal energyconsumption can commence. Experiments have shown that it is possible toachieve a mean error of even less than 15% in the estimation of energyconsumption, without information on the person's real maximum oxygenconsumption. The accuracy of the method is based on the successfuldetermination of respiratory frequency.

The method also does not necessitate calculation of the momentary ormean pulse frequency, but rather the contribution of respiration to thepulse signal can be determined directly, with the aid of the stamping ofthe pulses, which is described in greater detail later.

We use the term pulse period to refer to the period of time during whichthe heart actually beats, and during which there is a strong pulsesignal variation in the electrically measured pulse signal, caused bythe heartbeat. We use the term pulse interval to refer to the timebetween the successive pulse periods. Within this time there arevariations (noise), which are mainly influenced by respiration. Theperiodic contribution to pulse interval noise made by respiration can bedistinguished by the method according to the method

The determination taking place in the time domain is characterized bythe periodicity of the pulse interval noise being detected without aconversion of coordinates, for example, to the frequency plane. Thedetermination in the time domain can be made by collecting, with the aidof signal analysis, time stamps made on the basis of the detected pulseperiods, in order to detect the periodicity of the time stamp series.

We use the term parameter depicting the person's respiration to referprimarily to the respiratory frequency. However, by utilizing the basicidea of the invention together with preliminary data parametersdepicting the person, it is also possible, however, to determine themagnitude of ventilation.

In the following, various embodiments of the invention are describedwith reference to the accompanying drawings.

FIG. 1 shows a flow diagram of one embodiment of the present invention,

FIG. 2 shows a schematic diagram of one embodiment of the presentdevice, and

FIG. 3 shows part of a heart signal, by way of example.

FIG. 1 shows one possible way to implement the method according to theinvention. The measurement 101 of the pulse signal takes place typicallyelectrically using a pulse sensor, for example, a pulse belt securedaround the chest, or with the aid of separate electrodes placed on theskin. The pulse signal is obtained by analyzing the heart signal, inorder to detect the beats of the heart from the signal. The heart signalis illustrated in FIG. 3, in which the pulse period is marked with thereference number 32 and the rest period between the pulse periods withthe reference number 34. A temporally well-defined point, such as themaximum or zero point of the signal (in FIG. 3, maximum point 36) ispreferably detected from the pulse periods. Some method well known inthe field can be used in the detection. In FIG. 3, the pulse intervaldefined by these points is marked with the reference number 38.

If necessary, the pulse signal is transferred, in stage 102, over a wireor wirelessly, from the pulse sensor to the terminal device, in whichthe method stages 103-108 described below can be implemented. As statedlater, all the stages can also be performed in a single device.

In stage 103, time stamps corresponding to the heart bears are set onthe basis of each individual signal. In stage 104, a number series forfurther analysis is formed form these time signals.

In stage 105, the period of the series is determined from the timeseries formed in stage 104. This can be found, for example, bycalculating the second derivatives of the series and examining the zerosof this new series, i.e. the change points of the sign. One possible wayto implement this stage of the method is described later in greaterdetail in Example 1.

In stage 106, some property correlating statistically with respiration,typically respiratory frequency, respiration amplitude, or the amount ofair moved in respiration (ventilation), or several of these, aredetermined with the aid of the period of the series. An approximation ofthe respiratory frequency can be obtained directly on the basis of theperiod, whereas the calculation of the other variables typicallyrequires preliminary data.

We have observed that the periodicity of the pulse interval noiseutilized in the manner described above is a reliable indicator ofrespiration. A particularly advantageous feature of the method describedis that from only the electrically measured pulse signal a respirationsignal is also obtained with a good accuracy and by simple calculationtaking place in the time domain. A relatively short period of time canbe used as the monitoring interval, so that the parameter depictingrespiration can also be updated reasonably quickly. When a new pulse isregistered and time stamped, only a few calculation operations will berequired to calculate the updated period. A typical monitoring intervalof about 5-15 pulses at the pulse and respiration levels during exercisewill provide a first approximation for the momentary respiratoryfrequency. If necessary, this can be made more precise, at the expenseof the time resolution, by using a longer monitoring interval.

According to one highly preferred embodiment of the invention, aparameter depicting respiration is used to calculate energy consumptionduring exercise. In that case, typically at least one preliminary datum,either of the person who is the object of the measurement and/or of thesport being performed by him/her, will be used for assistance. Thepreliminary data comprise data that can be determined on the basis oftests or data that are not directly related to oxygen consumption. Theycan comprise, for example, the person's activity class, weight, height,or sex, or information on the nature of the sport being played by theperson. The term nature of the sport refers primarily to whether thesport in question is a sprint-type sport or an endurance sport. Theactivity class (typically on a scale of 1-10) can, in turn, bedetermined on the basis of, for instance, the amount of training theperson performs, without physical tests. Other personal orsport-specific data can also be used. In stage 108, the necessarycalculation can be performed, always depending on the availablepreliminary data and known parameter/s depicting respiration. Accordingto a greatly preferred embodiment, selected preliminary data are useddirectly as factors scaling the respiration parameter or parameters,which will further simplify and accelerate calculation. In thecalculation, the preliminary data can be given various weights. Thefinal result is preferably converted into absolute momentary values ofenergy consumption (e.g., kcal/min). The cumulative energy consumptionof the exercise can also be calculated. The consumption can also begiven as some relative values. In stage 109, the final result isdisplayed to the user.

Particularly in the starting or end stage of the exercise or othertraining, the respiratory frequency does not generally correlatedirectly with the energy consumption at that time. When a person startsexercise, their respiration does not immediately reach a levelcorresponding to the momentary energy consumption. On the other hand, atthe end of the exercise, or during a break in it, the respiratoryfrequency will remain high, even though the physical stress is over.These factors can, however, be taken into account by monitoring thetemporal change in the respiratory frequency, the heart rate, or someother measurable variable depicting the change in rhythm in theexercise. If, over a specified period of time, a change of a predefinedmagnitude is observed in such a variable, the respiratory frequency canbe corrected by calculation towards a respiration-rate value thatcorresponds better to the actual energy consumption. A real-timecorrection can take place, for example, by keeping the momentaryrespiratory frequencies in the buffer memory at predefined monitoringintervals, and comparing the latest respiratory frequency received withthe previous values. A more detailed depiction of the correction processcan be implemented in the manner shown in Example 2, but one skilled inthe art will understand that a calculation achieving a correspondingeffect can be implemented in very many different ways.

The correction of the energy-consumption value is preferably boosted.This means that the energy-consumption values are corrected relativelymore in relation to how much change occurs in the variable depicting thechange in rhythm of the exercise. This compensates, for example, for theslow change in respiration or pulse, relative to the momentary intensityof exercise. The variable depicting the change in rhythm can, of course,also be, for example, information received through an accelerationsensor, in which case it may not be necessary to boost the correction.

Even though the present document describes the use of energy-consumptioncalculation and the correction calculation based on changes inrespiratory frequency in connection with a respiratory-frequencydetermination taking place on a time domain, they can equally well beused in connection with other respiration-rate determination methods.Because the ways of calculation described can, however, also beimplemented with a small computing capacity aid in real time, aparticular advantage is achieved with their joint use.

The actual energy consumption is preferably calculated on the basis ofthe second-degree behaviour of the respiratory frequency, which has beenobserved to correspond well to the real energy consumption. Further,according to a preferred embodiment, in addition to respiratoryfrequency only general preliminary-data parameters that cannot bederived directly from metabolic tests concerning the person are used inthe calculation, as described above. For example, height and weight(mass) are direct quantities according to SI units, which are easy tomeasure and are generally already known to the user with good accuracy.The activity class can be defined with the aid of existing widely usedtables. These can be used directly as factors weighting the respiratoryfrequency or a secondary parameter calculated from it.

The known solutions approach the energy-consumption calculation problemfrom an entirely different direction; i.e. they measure or estimate themetabolism of the person, which is then used as a basis for thecalculation of energy consumption. The present solution, on the otherhand, is based on collecting sufficient general information the user, sothat a estimate can be made of the assumed respiration-energyconsumption dependence, This can also be regarded as being morereliable, in the sense that the effect on the final result of anindividual erroneously entered preliminary-data factor is smaller thanif, for example, an erroneously measured or entered VO2max measurementresult is used as the basis of the calculation. Thus in the mannerdisclosed it is possible to create a way to calculate energy consumptionthat is both pleasant for the user and reliable.

With reference to FIG. 2, the method disclosed is preferably performedeither entirely or partly in a portable device, preferably in a wristopdevice 220. The measurement of the heart signal can be performed withthe aid of a heart-rate belt 210, with the aid of the transmitter 214 inwhich the pulse signal is transferred by means of electromagneticradiation 250 to the stop device 220. The transfer can take placeinductively or with the aid of a radio-frequency signal. The measuringelectrodes of the heart-rate belt are marked with the reference number212. The wristop device 220 preferably comprises a pulse-signal receiver202 and a processing unit 204 for setting the time stamps of thepulse-interval periods and for defining in the time domain at least oneparameter depicting respiration, on the basis of the periodicity of thesignal containing the pulse-interval periods of the pulse signal. Inaddition, the device has a buffer memory 206, for storing pulse data,the time-stamp series, and/or the calculated respiratory parameters. Thesignal processing of the pulse signal and the necessary othercalculation are performed typically in a microprocessor, or in aseparate microcircuit designed for these functions. The device can alsohave memory for the longer-term storage of pulse, respiration, and/orenergy-consumption information. In addition, usually the device also hasa display 224, in which the result of the calculation can be displayed.

Thanks to the method disclosed, the processing unit 204 can be made tobe small and to consume little power.

The method can also be implemented in such a way that the calculation ofthe parameter depicting respiration can be performed either entirely orpartly in the heart-rate belt or in some other sensor device, in whichcase the whole pulse signal need not be transferred to the terminaldevice. Thus, it is sufficient if only the final result of thecalculation, or intermediate results together with their related timedata are transferred at intervals to the terminal device.

The method can be implemented during exercise or also afterwards in acomputer, to which the pulse signal or pulse data in the memory, thetime stamps or intermediate results calculated from them, or derivedparameters are transferred directly from the sensing device, orindirectly, for example, from a wristop computer.

The above detailed description of embodiments of the invention, theaccompanying drawings, and the following examples, do not restrict theinvention, but instead should be taken only as examples of ways toimplement the invention in practice. The invention should be interpretedin the full extent of the Claims and taking the Doctrine of Equivalentsinto account.

EXAMPLE 1

This example illustrates how the respiratory frequency can be determinedsimply in the time domain from the pulse signal. The starting point isthat the heartbeats have been detected from the pulse signal and withtheir aid a series of time points has been selected from the noise ofthe pulse-interval periods.

The time series picked out of the pulse signal and used in thecalculation of the respiratory frequency and/or ventilation is a seriesof numbers, which consist of time points. Each time point corresponds tothe moment when the heartbeat was detected. The time is measured, forexample, in milliseconds.

This provides a monotonously increasing number series of moments in time(in milliseconds, for example, 0, 1010, 1950, 2800, 3650, . . . ), withthe possible exception of the moment when as a result of the overflow ofa variable containing time stamps the reading is recommenced from zero.The approximation of the first derivative of the signal strengthscorresponding to this number series is (t2−t1)/1=sv1. The approximationof the second derivative is (sv2−sv1)/1, in which sv2=(t3−t2)/1. Byexamining the moment of change pf the sign of the second derivative anew number series is obtained: tt1, tt2, tt3, . . . In this numberseries, tt1=−the moment in time, when the sign became positive (ornegative)+the moment in time, when the sign became positive (ornegative) for the following time. This time period depicts theperiodicity of the pulse-interval noise, from which the respiratoryfrequency and ventilation can be approximated.

EXAMPLE 2

The correction of the respiratory frequency in order to take changes inthe rhythm of the performance into account (Stages 1-6) and further theenergy-consumption value (Stages 7-8) can be implemented for example, inthe following manner:

-   -   1. Calculate the momentary respiratory frequency    -   2. Search for the global minimum of the respiratory frequency        (in a specific period in time)    -   3. Search for the local maximum of the respiratory frequency (in        a specific shorter period in time)    -   4. If the new respiratory frequency is sufficiently less than        the local maximum (for example 15%), reduce the respiratory        frequency by a coefficient 0 . . . 1 (for example 0.7)    -   5. If the new respiratory frequency sufficiently exceeds the        local maximum (for example 20%), update the local maximum        towards the latest respiration-rate value    -   6. Deduct a correction factor, which can be a fixed value or a        value (offset correction) adapting with the aid of the global        respiration-rate minimum, from the respiratory frequency        pre-processed in Stages 1-5    -   7. Raise the respiratory frequency pre-processed in Stages 1-6        by the power of two    -   8. Scale the value obtained directly by the preliminary-data        parameters.

1. A method for monitoring the physiological state of a person duringexercise using a portable computerized device, comprising: monitoringthe heartbeat of the person in order to obtain heartbeat pulse signals,each heartbeat pulse signal having a maximum and minimum value and aduration or period, and wherein a pulse interval exists betweenconsecutive heartbeat pulse signals, and determining at least oneparameter depicting the respiration of the person on the basis of theperiodicity of the temporal variation of pulse data contained in thepulse interval, wherein the periodicity of the temporal variation of thepulse data is determined, using the portable computerized device, in thetime domain by time stamping pulse signal data and using the timestamped pulse data, and the parameter depicting respiration is used toestimate energy consumption of the person, and further comprisingestimating the energy consumption on the basis of a second degreebehavior of the parameter representing respiration.
 2. A portable devicefor monitoring the physiological state of a person during exercise,which device comprises: a sensor for detecting heartbeat in order tocreate heartbeat pulse signals or means for receiving heartbeat pulsesignals created with the aid of such sensor, each heartbeat pulse signalhaving a maximum and a minimum value and a duration or period, andwherein a pulse interval exists between consecutive heartbeat pulsesignals, and a processing unit for defining at least one parameterdepicting respiration, on the basis of the periodicity of the temporalvariation of pulse data contained in the pulse interval, wherein theprocessing unit is adapted to determine the periodicity of the temporalvariation of the pulse data by time stamping pulse signal data, and toestimate energy consumption of the person on the basis of the at leastone parameter depicting respiration, wherein the device is adapted toestimate the consumption of energy on the basis of a second-degreebehavior of respiration frequency.